Various kinds of rotary welding members made of synthetic resin material of the type mentioned above have been proposed (for example, Japanese Patent Publication No. SHO 57-36131, and Japanese Utility Model Public Disclosure Nos. SHO 60-62613 and 60-155654).
A conventional welding member made of resin for being rotated and welded to a resin surface such as, for example, a stud 1 (FIG. 1) and fastener 2 (FIG. 2) usually has such a structure as that of a post portion 3 and is provided with a disk-shaped flange portion 4 mounted upon one end thereof, and the flange portion 4 is provided with engaging projections 5 which project from the peripheral portion at diametrically opposite portions of a welding surface thereof.
In order to mount this member made of resin on a mounting base plate made of resin, the stud 1 or the fastener 2 is, as is shown in FIG. 3, held and pressed at the desired position on a mounting base plate 6 made of resin by means of a rotary jig 7, and a cut-out section 7a of the rotary jig 7 is engaged with each engaging projection 5 and rotated at a high rate of speed (FIG. 3). Both contacting surfaces of the flange portion 4 and the mounting base plate 6 are melted by frictional heat generated at that time. After the rotation has been stopped, the melting portion cools and hardens. As a result, the stud 1 or the fastener 2 is firmly secured to the mounting base plate 6.
However, in the stud 1 or the fastener 2 of the above-described structure, the contacting surfaces of the flange portion 4 and the mounting base plate 6 are melted by frictional heat generated when the rotary welding process is performed. In addition, if the mounting base plate 6 is not thick enough, even the rear surface of the mounting base plate 6 is softened by this frictional heat. As a result, as is shown in FIG. 4(a), the rear surface of the mounting base plate 6 is expanded outwardly by means of the pressure of the rotary jig 7 against the stud 1 or the fastener 2 for performing the welding process, or as is shown in FIG. 4(b), the rear surface of the mounting base plate 6 is warped or wrinkled because of the difference in the coefficient of contraction between the flange portion 4 and the mounting base plate 6 which are melted and welded together and then cool and harden, the difference resulting from the difference in the coefficient of linear expansion between the flange portion 4 and the mounting base plate 6. In other words, so-called "set-off" occurs.
Because of the foregoing, in the case where the rear surface of the mounting base plate 6 constitutes an outer surface of the product, the appearance is significantly spoiled by this set-off.